The invention relates to a method and to a device for determining the distance to an object.
In this connection, the measuring principle of the stimulation and evaluation of a sympathetic vibration is used: a transmitter transmits a modulated signal, in particular a modulated light signal, along a measured length in the direction of the object. The signal reflected by the object along the measured length or along a part thereof is detected by a receiver and deliveredxe2x80x94directly or via further componentsxe2x80x94to a filter module with defined phase frequency characteristics. The output signal of the filter module is in turn deliveredxe2x80x94directly or indirectlyxe2x80x94to the transmitter.
A resonant circuit is formed in this manner by the transmitter, by the measured length, by the receiver, by the filter component and, optionally, by further components. The transmitter is integrated in this resonant circuit such that its modulation frequency corresponds to the frequency of resonance of the resonant circuit dependent on the signal transit time over the measured length. The sought distance to the object can thus be calculated by a measurement of the frequency of resonance of the resonant circuit. The prerequisite for the functioning of this measurement principle is the use of a filter module whose phase response for the desired measurement range ensures the occurrence of a resonant condition, that is, the production of a phase shift of a total of 2xcfx80 or of 360xc2x0 or of an integral multiple thereof.
It is an underlying object of the invention to increase the accuracy of the distance determination for this known measurement principle.
Embodiments of the present invention provide a plurality of phase frequency characteristics, which differ with respect to phase and amount, used to form the resonant circuit and to determine the corresponding frequency of resonance. A comparison value is available by this taking into account of at least one further phase frequency characteristic which can be used in the manner explained in the following to check the determined distance and/or to increase the measurement accuracy.
The frequencies of resonance determined for the different phase frequency characteristics are preferably taken into account for the determination of the sought distance. For this purpose, the different frequencies of resonancexe2x80x94or the respective distances corresponding to these frequencies of resonancexe2x80x94are put in relation to one another. It is in any case important for the results of a plurality of measurements of the frequency of resonance for different phase frequency characteristics to be not further processed independently of one another, but to be combined with one another for the evaluation.
The evaluation of the measured frequencies of resonance is therefore not restricted to the purpose of an exclusive and direct distance determination. The frequencies of resonance determined for the different phase frequency characteristics can in particular be additionally used for an non-ambiguity check, i.e. for a check as to which of the possible resonant conditions is present. The phase information to be evaluated namely repeats for different scanning distances with a certain periodicity. With the known measurement principle, this can result in an incorrect determination of the sought distance under unfavorable conditions, that is, when the correct resonant condition was not used as the basis for the evaluation of the measured frequency of resonance.
In contrast to this, a check can be made in embodiments of the invention due to the taking into account of different phase frequency characteristics as to which integral multiple of 2xcfx80 or of 360xc2x0 is present. An extension of the range of non-ambiguity is thereby achieved.
A possible procedure for this non-ambiguity check comprises comparing the frequencies of resonance determined for the different phase frequency characteristics with one another or with at least one reference value, or of determining and comparing with one another a plurality of different phase frequency characteristics and in particular also distances corresponding to different resonant conditions. It is of advantage for such a comparison evaluation if the underlying distance of the object is unchanged or if any change in distance in the meantime is known and taken into account as well.
Alternatively or in addition to this non-ambiguity check, the measurement accuracy can be increased in that the different phase frequency characteristics, or the frequencies of resonance determined from these, are used to determine the parasitic phase shift, that is, that phase shift which is caused by components other than by the filter component explained.
As already explained, the resonant circuit can include further components, for example an amplifier, a band pass filter or a phase correction circuit. These components can change their contribution to the phase shift along the resonant circuit, for example due to temperature effects or aging effects, in a manner which cannot be predicted with sufficient accuracy by one-time reference measurements. Inaccuracies in the distance determination result if the portion of the measured phase shift which is due to the signal development along the actual measured length is no longer trackable.
To compensate for such errors and for further systematic errors, the plurality of frequencies of resonance corresponding to the different phase frequency characteristics can be used to determine the parasitic phase shift. This determination of the parasitic phase shift can take place individually for each distance measurement or the result can be compared with earlier results. To obtain the highest possible accuracy here, the distance of the object should remain as unchanged as possible, or any possibly known change in distance should also be taken into account, during the measurement of the different frequencies of resonance.
At least the different phase frequency characteristics, which correspond to the determined frequencies of resonance, and a suitable resonant condition can be taken into account for the explained determination of the parasitic phase shift.
An evaluation unit can be provided within the apparatus for the carrying out of the explained non-ambiguity check and/or for the explained determination of the parasitic phase shift.
The different phase frequency characteristics can differ with respect to their respective center frequency and/or to their gradient in the region of their different or same center frequency. It is, however, preferred for the transmission areas corresponding to the different phase frequency characteristics and respectively restricted by the 3 dB limit frequencies to overlap.
It is furthermore preferred if the phase shift caused by the phase frequency characteristic of the filter component dominates over the parasitic phase shift in the area of frequency of resonance of interest for the distance determination, in particular with the center frequency. The phase shift effected by the filter component can in particular be at least one and a half times larger than or twice as large as the parasitic phase shift.
It is furthermore of advantage if at least one phase frequency characteristic used for the determination of the distance is selected such that the corresponding frequency of resonance lies in a frequency range within which the frequency of resonance can be determined with particularly high measurement accuracy. The further frequencies of resonance can be selected such that the further evaluations, for example the non-ambiguity check or the determination of the parasitic phase shift, can take place particularly simply or with a particularly high accuracy.
As explained, it is of advantage if the different frequencies of resonance are determined for the respective same distance of the object. It is preferred for this purpose if the frequencies of resonance of the resonant circuit are determined simultaneously or at least at short time intervals to one another.
The constructional effort to realize the invention can be advantageously reduced in that a single filter module is provided with which the different phase reference characteristics can be selected. As an alternative to this, a separate filter module can be provided for each phase reference characteristic.
It is furthermore of advantage if a plurality of components are used commonly to form the different resonant circuits with the different frequencies of resonance. In particular the transmitter and the receiver are suitable for this in addition to the already mentioned filter module. In this way, the resonant circuits can therefore be designed parallel in part. The respectively individually provided part of the resonant circuits can be separated from the parallel part by a sum member or by a decoupler.
In accordance with an embodiment of the present invention, a filter module is made as a digital module. The phase reference characteristic of this filter module is thereby defined particularly accurately and reproducibly. Moreover, unwanted temperature effects and aging effects are excluded to a very large extent. Furthermore, with such a digital design, a plurality of different phase reference characteristics can be realized in a particularly simple and space-saving manner such that a partly parallel design of a plurality of resonant circuits is promoted.
It is preferred for a plurality, in particular all components of the apparatus, which form part of the resonant circuit and cause a phase shift, to be realized inside the digital module. A band pass filter and/or a phase correction circuit are in particular suitable for this additional integration.
Furthermore, this digital module can be made by an FPGA (field programmable gate array) or by a GA (gate array).
It must moreover be noted that the different embodiments of this further solution approach can be combined in any desired way with the solution approach first mentioned of the plurality of phase reference characteristics.
Embodiments of the invention can be used to particular advantage with respect to all solution approaches and embodiments for the determination of the object distance for the purpose of an automatic focusing of the optical transmission system and/or of the optical reception system of a barcode reader, since the accuracy of the distance determination is particularly important in a barcode reader with an autofocus function.
The invention will be explained in the following by way of example with reference to the drawings.